Astronomers, Using New Method, Find Evidence For Missing Matter

For years, scientists have been unable to account for all of the
material they believe would have been needed to form the cosmos
billions of years ago. But now two Johns Hopkins astrophysicists
may have found much of the "missing matter" by using a new method
to study the early universe.

Their new analytical method is detailed in a scientific paper to
be published on April 20 in the Astrophysical Journal. The
paper
was written by astrophysicists
Arthur F. Davidsen and HongGuang Bi.

"I have been very excited about this recent work," said David
Schramm, a University of Chicago astrophysicist involved in
similar research. "A long-standing problem in cosmology is,
'Where is all the normal matter?' Stars and galaxies do not add
up to as much normal matter as we feel must be there from our
analyses of nuclear processes that took place in the early
universe. Davidsen and Bi appear to have found the normal matter
out between the galaxies. Furthermore, the amount they find is
completely consistent with the amount we expected to be there
from our nuclear
physics arguments, so the whole picture holds together remarkably
well."

The dark-matter problem can be summarized like this: the universe
is made of visible matter and so-called dark matter. Visible
matter is seen in the form of stars and galaxies, which emit
light and other forms of radiation. Dark matter has not been seen
directly, but it is inferred to exist from the gravitational
effects it appears to exert on the visible matter.

Dark matter itself appears to come in at least two varieties. One
component is made of ordinary "baryonic" matter, the same stuff
that makes up all the visible matter in the universe. Baryons are
ordinary matter particles like protons and neutrons. But
astronomers have not been able to account for all the baryonic
matter that is thought to exist, based on their studies of the
nuclear reactions that occurred during the Big Bang. The visible
stars and galaxies contain only a small fraction of the total
amount of such ordinary baryonic matter believed to exist. The
other component of the dark matter is widely believed to be some
sort of exotic particle that does not emit or absorb light.

The analysis reported in the Hopkins paper suggests that the
missing baryonic matter has been found. It was spread throughout
intergalactic space in the form of a very diffuse gas of hydrogen
and helium atoms whose presence is detected through its effects
on light passing through it. These findings don't address the
nature and amount of the exotic type of dark matter, which
scientists believe makes up a majority of all matter in the
universe.

Astronomers had thought that the primordial medium of gases that
existed in the early universe was contained in individual
"clouds," with nearly empty space in between. But the Johns
Hopkins
astronomers have found evidence that the gases were not arranged
that way.
Using their method, Davidsen and Bi propose that the early
universe
contained a "continuous medium" of hydrogen and helium gases,
with regions of higher and lower density blending together
smoothly.

Although other scientists are using powerful supercomputers to
make similar calculations about the evolution of the universe,
the Johns Hopkins scientist have devised a method that requires
only "fairly simple analytical equations," Davidsen said. They
used their analytical method to explain data from observations
made by other astronomers over the past 20 years.

Astronomers have detected the primordial hydrogen gas by using
spectrographs to analyze light emitted by very distant objects
called quasars. Astronomers find places in the sky where there
are no galaxies, to get a clear line of sight to a quasar. As the
light from the quasar shines through space, it also shines
through the gas, like a headlight through fog. The quasar is so
far away that the light now reaching earth is from a time when
the universe was roughly one-quarter its present age, about 10
billion years ago.

But intense radiation from quasars and early galaxies has ionized
much of the gas, stripping away electrons from the atoms and
making the gas largely invisible to detection by spectroscopy. So
astronomers are only detecting a small portion of the gas.

"The gas is so highly ionized that we are seeing only the tail of
the dog," Davidsen said. "It's a big dog but we are only seeing
the tail. If we had a theory that told us exactly what dog it is,
based on what the tail looks like, then we could say something.
That's what we have now -- a theory that connects the tail to the
dog. We now believe we can say how much intergalactic gas,
baryonic material, there must have been."

Astronomers believe that the simplest elements, hydrogen, helium
and deuterium, were created in the Big Bang. Those simple
elements formed stars, in which the more complex elements were
manufactured. Exploding stars later released those more complex
elements.

But how did the hydrogen and helium come together to form stars
in the first place? Astronomers believe that concentrations of
the exotic form of dark matter formed gravity "wells" that
attracted the gases, beginning the process of star and galaxy
formation. The Johns Hopkins astronomers have used their method
to see that process going on in the universe about 10 billion
years ago, Davidsen said.

"Although a small fraction of baryons had by then managed to
condense into stars, galaxies, and quasars, it now appears that
most of them were still spread throughout intergalactic space, in
the form of very diffuse hydrogen and helium gas that was ionized
by the ultraviolet radiation of the quasars," Davidsen said.

The method was inspired by previous findings with the Hopkins
Ultraviolet Telescope, which was operated from the cargo bay of
a space shuttle in 1995. HUT observations of the primordial
helium yielded data that contradicted the theory that the
primordial gases were contained only in discrete clouds.

"The missing baryons used to be one of the so-called `dark matter
problems,' but this matter is no longer dark, thanks to the work
of Davidsen and Bi," Schramm said.

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